Three stars found circling Milky Way’s halo, study shows they were formed 13 billion years ago

In a new study, MIT researchers and undergraduate students spotted three of the oldest stars in the Milky Way galaxy. The stars were discovered in the “halo” region of the Milky Way. This region is a cloud of stars that envelopes the entire main galactic disk and stars found in this region are 12- 13 million years old, as per the team’s analysis.

The team of researchers named the three stars “SASS”, short for small accreted stellar system stars. The researchers believe each star once belonged to its own small, primitive galaxy that was later absorbed by the larger but still-growing Milky Way. The three stars found in the present time, are all that was left from their respective galaxies, as per researchers.

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The team suspects there may be more such stars available in the outskirts of the Milky Way.

The findings of the study were published on May 14 in the Monthly Notices of the Royal Astronomical Society journal. The study's co-authors are Mohammad Mardini, at Zarqa University, in Jordan; Hillary Andales '23; and current MIT undergraduates Ananda Santos and Casey Fienberg.

"These oldest stars should definitely be there, given what we know of galaxy formation," says MIT professor of physics Anna Frebel. "They are part of our cosmic family tree. And we now have a new way to find them."

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Researchers expect these SASS stars can help them as the analogues of ultra-faint dwarf galaxies, which are thought to be some of the universe’s surviving first galaxies.

"Now, we can look for more analogues in the Milky Way, that are much brighter, and study their chemical evolution without having to chase these extremely faint stars," Frebel says.

Three ancient stars found in the neighbourhood of Milky Way galaxy

The stars’ low chemical abundance hints that they were originally formed 12 to 13 billion years ago. Their low chemical signatures were similar to previously measured ultra-faint dwarf galaxies.

To get a clear picture, the scientists checked out the stars’ orbital patterns and how they move across the sky. The three stars are in different locations throughout the Milky Way’s halo and are estimated to be about 30,000 light years from Earth.

As they retraced each star's motion about the galactic centre, the team noticed a curious thing- relative to most of the stars in the main disk, which move like cars on a racetrack, all three stars seemed to be going the wrong way.

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In astronomy, this is known as "retrograde motion" and is a clue that an object was once "accreted," or drawn in from elsewhere.

"The only way you can have stars going the wrong way from the rest of the gang is if you threw them in the wrong way," Frebel says.

The fact that these three stars were orbiting in completely different ways from the rest of the galactic disk, combined with the fact that they held low chemical abundances, made a strong case that the stars were indeed ancient and once belonged to older, smaller dwarf galaxies that fell into the Milky Way at random angles.

(With inputs from agencies)